Remove fClient

Client (SPV) mode never got implemented entirely, and whatever part was already
working, is likely not been tested (or even executed at all) for the past two
years. This removes it entirely.

If we want an SPV implementation, I think we should first get the block chain
data structures to be encapsulated in a class implementing a standard interface,
and then writing an alternate implementation with SPV semantics.

src/net.h

@@ -86,7 +86,6 @@ enum threadId
     THREAD_MAX
 };
 
-extern bool fClient;
 extern bool fDiscover;
 extern bool fUseUPnP;
 extern uint64 nLocalServices;